net/http: allow Content-Type on 204 responses

Accidental change from fixing Content-Length on 204s
in http://golang.org/issue/6685 earlier.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/92400047

src/pkg/net/http/serve_test.go

@@ -2154,6 +2154,21 @@ func TestCodesPreventingContentTypeAndBody(t *testing.T) {
 	}
 }
 
+func TestContentTypeOkayOn204(t *testing.T) {
+	ht := newHandlerTest(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header().Set("Content-Length", "123") // suppressed
+		w.Header().Set("Content-Type", "foo/bar")
+		w.WriteHeader(204)
+	}))
+	got := ht.rawResponse("GET / HTTP/1.1")
+	if !strings.Contains(got, "Content-Type: foo/bar") {
+		t.Errorf("Response = %q; want Content-Type: foo/bar", got)
+	}
+	if strings.Contains(got, "Content-Length: 123") {
+		t.Errorf("Response = %q; don't want a Content-Length", got)
+	}
+}
+
 // Issue 6995
 // A server Handler can receive a Request, and then turn around and
 // give a copy of that Request.Body out to the Transport (e.g. any

src/pkg/net/http/server.go

@@ -799,18 +799,16 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 	}
 
 	code := w.status
-	if !bodyAllowedForStatus(code) {
-		// Must not have body.
-		// RFC 2616 section 10.3.5: "the response MUST NOT include other entity-headers"
-		for _, k := range []string{"Content-Type", "Content-Length", "Transfer-Encoding"} {
-			delHeader(k)
-		}
-	} else {
+	if bodyAllowedForStatus(code) {
 		// If no content type, apply sniffing algorithm to body.
 		_, haveType := header["Content-Type"]
 		if !haveType {
 			setHeader.contentType = DetectContentType(p)
 		}
+	} else {
+		for _, k := range suppressedHeaders(code) {
+			delHeader(k)
+		}
 	}
 
 	if _, ok := header["Date"]; !ok {

src/pkg/net/http/transfer.go

@@ -268,6 +268,22 @@ func bodyAllowedForStatus(status int) bool {
 	return true
 }
 
+var (
+	suppressedHeaders304    = []string{"Content-Type", "Content-Length", "Transfer-Encoding"}
+	suppressedHeadersNoBody = []string{"Content-Length", "Transfer-Encoding"}
+)
+
+func suppressedHeaders(status int) []string {
+	switch {
+	case status == 304:
+		// RFC 2616 section 10.3.5: "the response MUST NOT include other entity-headers"
+		return suppressedHeaders304
+	case !bodyAllowedForStatus(status):
+		return suppressedHeadersNoBody
+	}
+	return nil
+}
+
 // msg is *Request or *Response.
 func readTransfer(msg interface{}, r *bufio.Reader) (err error) {
 	t := &transferReader{RequestMethod: "GET"}